Want to own the chair on which Marie-Antoinette once sat? Knock on this Parisian door.

Numero 43, Rue de Monceau is an opulent town house in a quiet residential Paris neighborhood. There is nothing to tell you what goes on behind the massive wooden doors except a brass plaque with the name Kraemer.

It could be a private Swiss bank, which in a way is how the Kraemers have operated for five generations. Except the Kraemers sell 18th-century French furniture--Louis XIV, XV and XVI, meaning both the style and, very often, the pieces that belonged to the monarchs.

Museum directors know the Kraemers well; many 18th-century pieces in the Louvre, the Metropolitan and the Boston Museum of Fine Arts have passed through the hands of one Kraemer or another. Olivier Kraemer, who runs the business with his brother, Laurent, estimates that perhaps 10% of the people on The Forbes 400 list have come by No. 43, and half of them have bought something.

Like who, for instance? Kraemer smiles Swiss-bankishly and stays silent. Henry Kravis is believed to be one collector who buys from Kraemer, but Kravis won't confirm and neither will Kraemer.

"Kraemer is over-the-top discreet; one is always being reminded that one is never supposed to talk about the things one buys," says Gillian Wilson, a former curator who stocked the Getty Museum's collection with at least 20 pieces bought from the Kraemers over the course of 20 years. "There's always a lot of shuffling and muffling, and you always got the sense they were hiding someone more important than you in the next room."

Very often, they were. The Rothschilds stopped by frequently enough to be almost part of the family--Baron David de Rothschild handed Olivier Kraemer his Legion of Honor medal in 2003. Fiat
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)'s Gianni Agnelli was an old friend and customer. So was J. Paul Getty--a page in his diary from the 1930s reads, "I paid a visit to the young Kraemer, rue de Monceau." The youth in question was Olivier's and Laurent's late grandfather Raymond.

Henry Ford II rang on a cold New Year's Eve in 1965, in short order buying close to two dozen pieces that had taken the Kraemers a decade to assemble. Philippe, Olivier's father, finally said, "Mr. Ford, I think that's enough for one night."

Furniture making reached its zenith during France's ancien régime (15th to 18th centuries). For the first and probably the last time, a great desk and a great painting stood side by side as equals in artistry--and in cost. This was one-upmanship furniture, a trophy by which a young noble might outshine another.

The Kraemers sell the best of this and nothing else. "They're only interested in perfection," says Michael Andrew Wilson, an antiques scout who advises American collectors. "If there's a beautiful chair with a broken foot that needs to be restored, another dealer might spend $100,000 to restore it and turn around and sell the chair for $1 million. The Kraemers wouldn't touch that piece."

The effect of dozens of perfect pieces arranged over five floors of the Kraemer mansion is jaw-dropping. A writing desk by Jean-Henri Riesener made for Marie-Antoinette's Petit Trianon palace at Versailles has the sober lines of Louis XVI's neoclassical style. But all along the sides and up and down the fluted legs runs a riot of gilded flowers and leaves. The chasing is minutely carved, but it feels light, and it is: The government taxed gilt bronze by weight, and everybody wants to beat taxes, even a nobleman. You won't find a price tag on the desk, but expect to pay in the neighborhood of $13 million.

As important as ornamentation is proportion. Tucked away on an upper floor is a small wooden box--the French call it a niche--about 20 inches high, painted to resemble a theater with drawn red curtains flanking a small opening where the stage would be. Some ducal dog slept here in 1775. It is impeccable, more in the harmony of its lines than the fineness of its detail. So how much does an antique doghouse run? For the Kraemers, this clearly is the distasteful part of being a merchant in your own museum. "They only want people who understand what they've got," says New York architect and designer Peter Marino, whose clients include FORBES rich listers Bruce Kovner and Sydell Miller (she fell off the list in 2000). If you must know, the niche is about $150,000.

"One centimeter higher or wider and it just wouldn't give you the same pleasure," says Olivier Kraemer, looking at it fondly. "I've inherited this sense of proportion--we all have it in the family. I can look at a piece from 15 feet away and tell instantly if we're going to buy it or not."

That inner radar descends from Lucien Kraemer, Olivier's and Laurent's great-grandfather, who moved to Paris to make his fortune around 1870. By the time Lucien's son, Raymond, took over the business, Kraemer was already the preeminent dealer in the field.

As a prominent Jewish family, the Kraemers spent WWII hiding in the south of France. They returned to find the house at 43, rue Monceau intact but empty. Reichsmarschall Hermann Göring had commandeered the entire inventory. He only got a small part of it, however. The Gestapo sold the rest and pocketed the profits.

"My father had a little money left that we hadn't spent during the war," remembers Philippe Kraemer, Raymond's son. "He used it to buy one piece--a small Louis XV bureau known as a dos d'âne, a donkey's back. It was much more important to buy one really good piece than several that were less good. From there it took us another 15 years to build the business back up."

In generation five, Olivier's son Mikael, and Laurent's daughter, Sandra, are waiting in the wings. They have their work cut out. French 18th-century furniture prices have languished, even as prices for industrial furniture from the 1950s have soared. The hedge fund marquises and duchesses of today compete by spending their fortunes on contemporary art.

Which is why the Kraemers have just unveiled the first of several new galleries designed to show off their Louis against modern art and decor. "The styles work beautifully together. Who the hell wants to live in a period setting anyway? It's so low-iq!" says architect Marino. Marino bought a Louis XVI commode by Leleu from Kraemer for his own contemporary apartment. He pauses a beat. "I had to sell two of my children to pay for it."